Variation of the Sterical Properties of the N-Heterocyclic Carbene Coligand in Thermally Triggerable Ruthenium-Based Olefin Metathesis Precatalysts/Initiators

Abstract

A series of ruthenium complexes based on the kappa(2)( C,N) - (2- (benzo [h] quinolin-10-yl)methylidene ruthenium dichloride fragment featuring different neutral coligands L (L = 1,3-bis (2,6-diis opropylp henyl) -4, S- dihydroimidazol-2-ylidene (SIPr), 1,3-bis(2,4,6-trimethylpheny1)-4,5-dihydroimidazol-2-ylidene (SIMcs), 1,3-bis(2,4-dimethylpheny1)-4,5-dihydroimidazol-2-ylidene (81Xyl), and 1,3-bis(2-methylpheny1)4,5-dihydroimidazol-2-ylidene (SITO1)) was prepared, characterized, and tested in the thermally induced ring-opening metathesis polymerization of dicydopentadiene. In addition, the corresponding tricydohe#1phosphine derivative was investigated for comparison. All compounds were isolated as their trans-clichloro isomers. NMR spectroscopic features as well as structural features are, particularly within the NHC-bearing complexes, very similar, but their polymerization activity at elevated temperatures is distinctly different. While the SIMes derivatiie shows the desired properties, i.e., latency at room temperature and pronouneed polymerization activity at elevated temperature, 'all other preinitiators do not. The preinitiator featuring the SIPr coligarid is the most latent one, needing temperatures > 140 degrees C to show moderate: activity in the polymerization of dicydopentadiene. Compounds bearing the smaller N-heterocyclic carbene congeners are stable and latent at room temperature, but decompose upon heating, diminishing the polymerization activity at elevated temperatures. Density functional calculations show that the SIMes -derivative is the easiest to activate and yields the most stable 14-electron intermediate. Finally calculations reveal a distinct influence of the nature Of the N-heterocyclic carbene ligand on the position of the equilibrium of cis- and trans-dichloro isbrhers of the Complexes. -While the SLPr and the SIMes derivatives prefer the cisconfiguration, all other derivatives favor, at least in solvents with low dielectric constants, the trans-configuration. These computational findings were supported by the isolation and full characterization of the cis-dichloro isomer of the SIMes-bearing preinitiator obtained upon heating of its trans-isomer at 140 degrees C

Tuning the Latent Behavior of Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes in Dicyclopentadiene Polymerization

“…The free triflate counterion is severely disordered, and also two independent conformers are present in the asymmetric unit. The Mo(1A)−N(4A) bond distance is 229.2 (7) pm and suggests that N-chelation is sustained. The olefinic C(41A)−C(42A) bond distance is 140.0(11) pm, which indicates a pronounced double-bond character.…”

“…Latent catalysts should be tolerant toward all components of the polymerization mixture as well as toward decomposition or thermal degradation and, ideally, toward air. Generally, latent catalysts or precatalysts for ROMP can be activated by light, heat, acids, or mechanical force. ,− So far, most precatalysts for the ROMP of DCPD are based on ruthenium, due to their tolerance toward moisture and protic groups. , Generally, latency in the well-defined ruthenium alkylidene catalysts is achieved with chelating alkylidenes such as N -chelating (pyridyl, Schiff base, quinoline, quinoxalines), − ,− O -chelating (ether, carboxylate), ,− and S -chelating (thioether) , structural motifs.…”

Conformational Control of Initiation Rate in Hoveyda–Grubbs Precatalysts

“…Other small structural differences are also noted, such as slightly longer Ru–Cl bonds in the equatorial isomer and a shorter RuC bond (Ru–C28) in the axial isomer. The difference in Ru–O bond length between the conformational isomers is most likely due to the steric strain imposed due to the conformation of the cyclohexyl ether; some of the smaller differences may result from additional interactions arising from crystal packing …”

Cross-linked ROMP polymers based on odourless dicyclopentadiene derivatives

“…Ring opening metathesis polymerization (ROMP) reactions pose many advantages, such as high functional group tolerance, wide variability of monomers and solvents, low polydispersity of the products and, depending on the initiators used, high stereoselectivity, [16][17][18][19][20][21][22] sequence control [23][24][25][26][27][28][29] and latency. [30][31][32][33][34][35][36][37][38][39] The usefulness of ROMP has been demonstrated in several applications, such as for surface coating, [40][41][42][43][44] bio-renewable materials [45][46][47] and even for drug delivery. [48][49][50] Amongst the industrial polymers produced by ROMP, the most promising and widely used monomer, is endo-dicyclopentadiene (DCPD) (1).…”